1. Field of the Invention
The present invention relates to food processing and, more particularly, to a process for preparing meals ready to eat using both pulsed electric fields and rotational retorting.
2. Description of Related Art
Ready-to-eat meals are gaining in popularity. In addition to "microwavable" meals for conventional home use, such meals have long been used by the military for feeding troops in the field. The military refer to these ready-to-eat meals as Meals-Ready-to-Eat or MREs. MREs and other ready-to-eat meals typically are processed in a manner which provides them with a long shelf life without refrigeration. This makes the meal very convenient, particularly for military applications. MREs and other ready-to-eat meals (hereinafter collectively referred to as MREs) are contained in "retort pouches" or other containers. These containers are filled with food and then processed to kill any microorganisms, such as bacteria and the like, and enzymes which cause the food to spoil. Currently, microorganisms and enzymes are killed using a retort sterilization process.
Briefly, a retort sterilization process involves heating the food to a high temperature for a long period of time. For example, beef stew may be retort sterilized at a temperature between 248.degree. F. and 252.degree. F. for a time between 24 minutes to 20 minutes, respectively. A discussion of the retort sterilization process is provided in U.S. Pat. No. 5,283,033 to Dodrill. The contents of this reference are incorporated herein by reference. In addition to the sterilization, the retort process also "cooks" the food so that it obtains the desired sensory characteristics (taste, texture, appearance, etc.) so that the food may be eaten directly from the pouch either cold or heated. The desired sensory attributes may be achieved at a high heat in only a short time. The sterilization process, however, involves applying the high temperature for a long period of time, as described above. As a result of this prolonged exposure to high heat, the retort sterilization process has several drawbacks: (1) the sensory aspects of the food are adversely affected; and (2) the nutritional value of the food is adversely affected. In effect, the food is overcooked.
Regarding the sensory aspects of the food, studies performed by the Center For Advanced Food Technologies (CAFT) found that retort sterilization reduced the firmness, springiness, and denseness and increased the flakiness of beef cubes in gravy. Note that beef stew is a common entree for troops' MREs. FIGS. 1A-1D are graphs illustrating some of these findings.
Regarding the nutritional aspects of the food, retort sterilization has been found to adversely affect certain thermally sensitive nutrients. For example, the CAFT found that retort sterilization greatly decreases the ability of carrots (an important ingredient of beef stew) to retain alpha and beta carotene, important antioxidants. FIG. 2 is a graph illustrating these findings.
Rotational retort sterilization has been found to provide advantages over conventional retort sterilization. "Rotation" refers to the agitation of the food within the container at a particular rate. The (Combat Rations Advanced Manufacturing Technology Demonstration (CRAMTD, now known as Food Manufacturing Technology) found that rotational retorting provides the following advantages:
1. It increases the heat transfer rate and thus reduces the length of the retort process, improves the sensory quality of food, and retention of nutrients; PA1 2. End-over-end agitations are superior to other types of agitation; and PA1 3. Application of agitation should take into consideration the particular food and product container.
Thus, even with the improvements provided by rotational retort processing the current retort sterilization process does not provide a satisfactory food product. Rather, retort sterilized foods tend to have poor sensory appeal, including taste and texture, and reduced nutritional value. Thus, there is a need to reduce, but not eliminate, the time and temperature for providing MREs in order to obtain the desired the sensory aspects and the nutritional value of the food without "overcooking" the food.
Pulsed electric fields (PEF) have been shown to be effective in inactivating microorganisms and in denaturing enzymes harmful to foods. PEF, described in detail below, is a non-thermal process which has no adverse effects on the food being processed. PEF is the introduction to food using short blasts of high voltage electricity. The PEF process typically is performed at refrigerated temperatures or room temperature, depending on the foodstuff, rather than elevated temperatures such as in the retort sterilization process. Because the PEF process does not heat the food, the food does not obtain the desired sensory characteristics. The resultant product of the PEF process is not ready to eat.
FIG. 3 is a block diagram of a typical PEF device 300. A pulsed power supply 302 generates a voltage. The voltage is stored in a capacitor 304 which discharges the electricity very rapidly to a first electrode 306. A second electrode 308 is connected to ground. When the voltage is discharged by the capacitor 304, an electric field is generated between the electrodes 306, 308. Between the first and second electrodes is a treatment chamber 310 which contains the food being processed. The electric field inactivates the microorganisms in the food. A more detailed description of a PEF device is provided in U.S. Pat. No. 5,549,041 to Zhang, et al. The contents of this reference are incorporated herein by reference.
The elimination of microorganisms by PEF is a flnction of the electric field intensity, treatment time (i.e., the pulse duration x the number of pulses), temperature of the food, microorganism concentration. PEF has been successfully demonstrated to substantially eliminate microorganisms in "pumpable" foods such as diary products, juices, and fluid eggs. In addition, the process of using PEF on solid and semi-solid foods is described in the Zhang patent described above.
There is a need to provide a process for making MREs which both sterilizes the food for long shelf life without refrigeration and which also provides improved sensory characteristics and nutritional content.